It’s common knowledge that most lenses are at their best (i.e. sharpest) between f/5.6 and f/8, depending on the lens. But why is that? Why does stopping down further actually make the image softer? This is what YouTuber ZY Productions explains in the video above.
As many photographers will already know, the reason a lens is softer at f/22 than it is at f/8 is due to a phenomenon called diffraction. But even if you know that—and use it out in the field—there’s a chance you’ve never actually dived into the subject so you could understand what’s happening at the optical level.
ZY Productions wants to fill in that knowledge gap, explaining the physics behind diffraction, why it affects your images, and how it affects your images.
The two most interesting points worth highlighting are (IMO) that (a) lenses get sharper as you stop down because stopping down reduces aberration, even while it increases diffraction, and (b) it’s only when the “blurry points” caused by diffraction become bigger than an individual pixel that you’ll begin to see the effect in your images. This has two consequences that are actually noticeable in the real world:
- All other things being equal, a higher-resolution sensor will show the effects of diffraction sooner, because the individual pixels are smaller.
- A really well-corrected lens will begin showing the negative effects of diffraction earlier. You can see this in Imaging Resource’s review of Nikon’s 14-30mm f/4S lens for the Z-mount.
But don’t take our word for it. Check out the video above to see a few examples and diagrams that explain all of this in more detail. And if you really want to get nerdy, grab your favorite lens, point it at a highly-textured surface like your couch, and see the impact of diffraction for yourself as you stop down.